I am confused as to why you would start with Child Protective Services.
You state your daughter-in-law and grandson have not contacted you as you expected. What about your son? Is he still living and in the picture? Have you contacted him to see if he has any information?
If your son is no longer living or has no contact with your family, then I suggest you call the Highway Patrol (1-800-TELL-CHP) to explain the situation and ask for assistance.
They should be able to tell you what other steps you can take in your search.
Good luck.